Incidentally, chapter 13 of the movie manga (I fucking love it) just got translated and actually does give an explanation for this.

I can't promise you'll accept the explanation, but it's an explanation nontheless.

That the one with STB? Anyways, she took a ton of damage in the movie manga, although it never seemed to slow her down...

But in the movie itself, that's not really the case. She blocked phalanx with a single shield while being binded by both hands and wasn't out of breath in the slightest. Fate actually disarmed one of her hands from the bind to block Nanoha's Divine Buster, and it took a heck of a lot more from her in order to block it.

They're two different continuities. Movie manga did it best.

Edit: Just read the chapter. So RH blocked all the damage for Nanoha without her having to control the shield herself, eh? Amusing.